Within the field of adhesives, pressure-sensitive adhesives (PSAs) are notable in particular for their permanent tack. A material which has permanent tack must at any given point in time have an appropriate combination of adhesive and cohesive properties. This distinguishes it, for example, from reactive adhesives, which in the unreacted state offer virtually no cohesion. For good product properties it is appropriate to adjust PSAs in such a way that the balance of adhesive and cohesive properties is at an optimum. This balance is typically achieved by converting polymer chains present in PSA formulations into wide-meshed networks. The nature of this network has a critical influence on the adhesive and cohesive properties of the PSA. A material featuring pronounced crosslinking, although having good cohesion, nevertheless has reduced pliancy, so that the material is unable to adapt adequately to the roughness of a substrate surface. Moreover, a material featuring pronounced crosslinking has only a relatively low ability to dissipate deformation energy such as occurs under load. Both phenomena reduce the bond strength. A material with a low level of crosslinking, in contrast, although able to flow on rough surfaces and to dissipate deformation energy, with the consequence that the adhesion requirements may be met, is nevertheless inadequate in its resistance to load, owing to a reduced cohesion.
One kind of crosslinking which has an effect on the adhesion/cohesion balance is temporary polymer-chain interlooping. However, this is sufficient for adequate cohesion of the PSA only when the molar mass of the polymers is sufficiently high. PSAs based on natural rubbers may rest solely on this crosslinking principle. Further possibilities of setting the crosslinking of the PSA are chemical crosslinks, which are therefore irreversible. Chemical crosslinking can also be achieved by means of radiation treatment of the PSAs. Another possibility is to utilize physical crosslinking principles. Examples of such crosslinks, typically thermoreversible, in PSAs are present in thermoplastic elastomers, such as in certain block copolymers or semicrystalline polymers.
Besides the crosslinking principles referred to, it is also possible to use fillers for raising the cohesion. In that case a combination of filler/filler interactions and filler/polymer interactions frequently leads to the desired reinforcement of the polymer matrix. A raising of cohesion based thereon represents a further physical crosslinking variety.
For fillers which are mentioned with a view to a reinforcing effect in PSAs, the class of the pyrogenic (or fumed) silicas deserves particular mention. These silicas are used, inter alia, as thickeners, gelling agents or thixotropic agents in a very wide variety of fluids, utilizing their effect on the rheological properties of the fluids. The use of hydrophilic and of hydrophobic silica is described in this context. Examples of the use of pyrogenic silica in the field of PSAs are described in US 20060205835 by tesa AG, in U.S. Pat. No. 4,710,536 by 3M and in EP 108 32 04 B1 by Dow Corning.
As further fillers, the use of modified phyllosilicates for improving product properties has been described in U.S. Pat. No. 7,144,928 by tesa AG, in WO 02/24756 A2 by Rohm & Haas and in JP 2002 167, 557 by Sekisui.
In all of these cases the reinforcement results from the effect of the particles on the elasticity modulus of the elastomer composite. The interaction in this case is brought about by physical interactions between individual particles, on the one hand, and between particles and polymers, on the other. Often, however, these physical interactions are not enough to withstand even low mechanical deformations, such as may occur, for example, when a PSA joint is loaded by shearing or peeling. This nonlinear phenomenon is known as the Payne effect and is manifested as a loss of elasticity modulus under deformation. A review of the description of this effect and of various approaches as a mechanistic explanation is given by Heinrich and Klüppel [G. Heinrich, M. Klüppel, Adv. Polym. Sci., 2002, 160, 1-44].
In the preceding section, a variety of examples have been given of types of crosslinking that may be employed in PSAs for improving the product properties, especially the cohesion. For each of these varieties of crosslinking, the question arises of to what extent they affect the processing properties, and more particularly the coating characteristics. This is debated below.
Besides the product properties and hence the optimum balance of adhesive and cohesive properties in a PSA, its processing properties are also of central importance. Generally speaking, the processing properties of a formulation are reduced by its crosslinking. In a majority of cases indeed, processing becomes impossible. It is therefore advantageous to carry out or to initiate crosslinking not until during or after processing, and in particular during or after coating. However, where the crosslinking state results from the mere presence of a constituent in the formulation, as is the case with the abovementioned fillers, then the processing characteristics are adversely affected by its very presence. Polymers with high molar masses are likewise among formulation constituents which by virtue of their state of interlooping have advantageous product properties and yet, likewise owing to their state of interlooping, may show disadvantages in processing properties. In both cases, namely both interlooping and fillers, the physical principles which lead to the crosslinking of the PSA system and hence to advantageous product properties have negative consequences for the processing characteristics, particularly the coatability.
Traditional approaches to escaping this dilemma have been based on the use of solvents as operating assistants. An increased environmental awareness and the desire for ever more efficient production techniques, however, are underlying the trend toward solvent-free operations. In comparison to solvent processing methods, the polymer-based PSA base compositions, in the case of the hotmelt processes have a state of crosslinking in their melt, as a result of the interlooping and/or filler particles, which is associated with significantly higher viscosities and elasticities.
In contrast to physical modes of crosslinking, chemical crosslinking methods afford the formation of a network which can be initiated by an appropriate operating regime only during processing. However, the use of chemical crosslinkers is limited by their pot-life reactivity. If the network forms in too pronounced a way before the material has been coated, the elasticity increase which has already taken place results in a deterioration in the processing properties, and reduced-quality coating outcomes may result. One particular difficulty arises in the case of solvent-free systems, since, here, elevated temperatures are necessary for processing, leading at the same time to an acceleration of the chemical crosslinking reaction. One example of a system of this kind is described in US 20050129936 A1 by tesa AG. Radiation crosslinking methods appear advantageous in this context, since only after coating is the formation of a network initiated deliberately, as proposed for example in EP 153 21 82 A1 by tesa AG and EP 167 68 70 A1 by National Starch. However, in order to obtain networks having a structure satisfying the subsequent product requirements in respect of shear strength, polymers of decidedly high molar mass are needed, which in turn, as a result of their state of interlooping, may have disadvantages in terms of processing characteristics.
Typically, the processing properties of a material deteriorate as its elasticity goes up. Formation of a network always leads to an increase in the storage modulus and hence to upper elasticity. Consequently, there is a deterioration in the fluidity, which is needed for processing of the coating, or even a complete loss of fluidity. In the case of coating, then, inhomogeneities may occur in the coating outcome, possibly going as far as melt fracture. A variety of authors describe this phenomena, especially for capillary dies and extrusion dies. Literature references on this can be found in Pahl et al. [M. Pahl, W. Gleiβle, H.-M. Laun, Praktische Rheologie der Kunststoffe und Elastomere, 4th ed., 1995, VDI Verlag, Düsseldorf, p. 191f] and Tanner [R. I. Tanner, Engineering Rheology, 2nd ed., 2000, Oxford University Press, Oxford, p. 523f].
Systems are therefore sought which preferably can be coated without solvent and which exhibit a combination of good product properties on the one hand—and here particularly in respect of cohesion—and improved processing properties on the other, especially coatability.
One particularly advantageous example of systems which at least partly satisfy this combination of requirements is represented by block copolymers comprising segments which soften at high temperatures (known as the hard phase) and others which at application temperature are present in melted form. The softening temperature of the hard phase is typically adjusted, through the use of specific monomers, such that good product properties prevail at room temperature and yet at temperatures that are rational from an operational standpoint the material can easily be coated from the melt. Since these materials typically do not have high molar masses, their melt viscosity and elasticity, as soon as the hard phase is in softened form, are comparatively low.
A disadvantage of the above-discussed PSAs based on block copolymers, however, is their thermal shear strength, which is limited by the softening of the hard domains that sets in at an elevated temperature. A further disadvantage to be cited are the costly and inconvenient preparation conditions for block copolymers. In order to be able to prepare polymers having the requisite blocklike structure in sufficient quality, controlled or living polymerization techniques are necessary, some of which are complex. Moreover, not all monomer combinations can always be easily realized. Hence the block copolymer approach, on the one hand, therefore, is seen as not being universally flexible for numerous polymer systems. On the other hand there is a need for PSAs having better thermal shear strength.
Surface-functionalized fillers, more particularly the classes of the fumed and precipitated silicas, are likewise described in US 2006/0035087 A1 of NanoProducts Corp., US 2006/0204528 A1 of Byk-Chemie GmbH, and WO 2007/024838 of E. I. du Pont de Nemours and Co., in order to construct a chemical network. Crosslinking in those cases takes place through formation of covalent bonds, the crosslinking reactions being initiated thermally. This in turn may be problematic when the fillers are incorporated, and when the composites are processed, since, owing to elevated temperatures, crosslinking may commence during the actual operation. DE 10 2005 022 782 A1 of tesa AG describes functionalized particles which, on exposure to electromagnetic radiation and/or particulate radiation, lead to crosslinking of the pressure-sensitive adhesive; however, there continues to be a need for thermally initiable crosslinking methods for pressure-sensitive adhesives which can nevertheless be processed in a hotmelt process.
It is therefore an objective of the present invention to provide a flexible scheme which encompasses a suitable combination of material and process so that it is possible to prepare PSAs which can preferably be processed without solvent and which have good processing properties, such as, for example, an increased pot life for the crosslinking reaction, and good product properties.